Absorbent resins which are excellent in water absorbability and retainability and are capable of absorbing and retaining a large amount of water have been developed in recent years and utilized generally as sanitary napkins or disposable diapers in the field of sanitary goods, water retaining agents, etc., in the agricultural and horticultural fields, as well as sludge coagulants, dewing inhibitors, water stoppers, etc., in the civil engineering and constructing fields.
For such absorbent resins, there have been proposed, for example, hydrolyzate of starch-acrylonitrile graft polymers, starch-acrylic acid graft polymers, hydrolyzate of vinyl acetate-acrylate ester copolymers, crosslinking products of polyacrylic acid salts, carboxymethylcellulose, etc.
Generally, the performance of an absorbent resin is evaluated based on the absorption amount, absorption rate, gel strength in a swollen state, etc. Among them, there have been made various proposals for the improvement of the absorption performance such as the absorption amount and the absorption rate (for example, reference is made to Japanese Patent Application Laying Open Nos. Sho 57-158210, Sho 59-62665, Sho 61-97301, etc). While on the other hand, the gel strength upon swelling has an inverse relationship with the absorption amounts thus showing a trend in which the absorption amount is lowered as the strength is increased. It is considered that an ideal absorbent resin should be capable of satisfying both the absorption performance and the gel strength upon swelling as described above, but at the present time, no such resin has yet been obtained.
When the absorbent resin is swollen into a gel-like state upon absorbing water, there are problems in that it suffers from dynamic forces such as swelling pressure, as well as the gel-like resin being degraded due to circumstantial factors such as exposure to high temperature or sunlight and, further, due to the effect of oxygen in the air. In an extreme case, the swollen gel-like state can be maintained only within several hours and the function of water absorbability and retainability is lost. Similar degradation also occurs in the absorbent resin swollen by the absorption of urine, failing to retain the state of the swollen gel depending on the case. Such a phenomenon causes significant trouble upon using the absorbent resin in various application uses and the development for an absorbent resin of excellent aging stability for swollen gel has been demanded.